Microscopic examination

Using a cross-section to provide orientation, surfaces can subsequently be split or shaved along the radial or tangential planes. For microscopic examination, tissue sections must be accurately cut along radial, tangential or transverse surfaces using a razor blade or comparably sharp instrument. When working on objects, it is sometimes possible to cut sections from furniture parts directly. In other cases it is more expedient to dissect first a tiny piece from the object, which can then be further prepared as required. Typically, a fragment 3X3X10 mm will be sufficient. To obtain the best results it is normally advisable to soften this initial sample by boiling to enable very thin slices of wood tissue from each of the three principal planes to be taken for microscopic examination. These should normally be approximately 3X3 mm and as thin as possible. Ideally, the section should be thinner than the diameter of the smallest cells, that is less than 0.020 mm (i.e. 20 pm). It is possible to achieve such thin cuts using a razor blade but more consistently even results can be obtained with a microtome. In reality, sections cut by hand will be variable in thickness, and some portions of the sample will be too thick to show detail. Useful cellular detail will usually be found along thin edges of hand-cut sections. The section is placed on a glass microscope slide, moistened with a drop of water, and then covered with a thin cover glass. When placed on the stage of a standard compound light microscope, the translucent section is illuminated with transmitted light and the cellular detail can be examined at magnification in the range of 100X—500X. Ideally, the microscope should have facilities for using polarized light as this is useful for identifying crystals (for choice of a suitable instrument see Hoadley, 1990; also Normand, 1972; Core et al, 1979). Samples can be stained or bleached to enhance the microscopic image of the wood section (Blanchette, 1992a; Florian et al, 1990; Grosser, 1977; Walter, 1980). The stain Safranin 'O' is commonly used to increase contrast of wood section. Sections can be permanently mounted for safe storage and long term use (Florian et al, 1990).

Among the conifers, microscopic details offer the most distinctive differences from one species to another and thus provide the most reliable basis for separation. Among the routinely useful features are the height and width of the rays, the types of ray cells present, the shape and number of cell-wall pits (voids in the cell walls connecting adjacent cells), the smoothness of the cell walls, the presence and colour of contents of the cells, or spiral thickenings in the tracheids. Such detail is often quick and easy to see and evaluate. For example, among the pines, the cell walls of ray tra-cheids are smooth in the soft pines (such as eastern white pine, Pinus strobus), providing a certain separation from the hard pines (such as Scots pine, Pinus sylvestris), which have dentate ray tracheids with jagged walls (see Figure 2.5b). As another example, the characteristic spiral thickenings observed in yew, Taxus spp. (Figure 2.9) quickly separate this wood from other conifers found in furniture.

Figure 2.10 The scalariform perforation plate (the ladder-like feature also seen in Figure 2.4b) in combination with the numerous tiny pits (minute alternate inter-vessel pitting) are distinguishing features of birch (Betula spp.)

Figure 2.9 Tangential microscopic view showing spiral thickening in tracheids in yew. This distinctive feature, along with the absence of both resin canals and longitudinal parenchyma, distinguishes yew from other conifers

Figure 2.10 The scalariform perforation plate (the ladder-like feature also seen in Figure 2.4b) in combination with the numerous tiny pits (minute alternate inter-vessel pitting) are distinguishing features of birch (Betula spp.)

Among the hardwoods — the diffuse-porous hardwoods especially — microscopic analysis also provides the best means of confirming many genera, and in most cases, the only way of separating species within a genus. For example, in Betula (birch), the multiple grate like end walls (scalariform perforation plate) separating consecutive vessel elements together with the numerous tiny pits (minute alternate inter-vessel pitting) laterally connecting vessels, are distinguishable features of this genus (Figure 2.10).

Figure 2.11 Radial microscopic views of black walnut: (a) shows crystals in longitudinal parenchyma cells; (b) shows reticulate thickenings, called 'gash-like pits', in late-wood vessel elements. These two features can be used to separate North American black walnut (Juglans nigra) from European walnut (Juglans regia) as they are present only in the North American species

Figure 2.11 Radial microscopic views of black walnut: (a) shows crystals in longitudinal parenchyma cells; (b) shows reticulate thickenings, called 'gash-like pits', in late-wood vessel elements. These two features can be used to separate North American black walnut (Juglans nigra) from European walnut (Juglans regia) as they are present only in the North American species

Table 2.2 List of microscopic features for the identification of hardwoods

Solitary vessel outline angular

Perforation plates — simple or scalariform including number of bars or other types Intervessel pits arrangement and size intervessel pits scalariform or opposite or alternate or polygonal intervessel pits minute or small or medium or large range of intervessel pit size Vestured pits present Vessel ray pitting with distinct borders or with much reduced borders, pits rounded or angular or with much reduced borders, pits horizontal to vertical or of two distinct sizes or types in the same ray cell or pits unilaterally compound and coarse (over 10 pm) or pits restricted to marginal rows Spiral thickenings present throughout body of vessel element or only in vessel element tails or only in narrower vessel elements Mean tangential diameter of vessel lumina " 50 pm or 50—100 pm or 100—200 pm or > 200 pm vessels of two distinct diameter classes, wood not ring-porous Vessels per mm2 - " 5 or 5-20 or 20-40 or 40-100 or > 100 Mean vessel element length - " 350 pm or 350-800 pm or > 800 pm Tyloses and deposits in vessels tyloses common or sclerotic or gums and other deposits in heartwood vessels Wood vessel-less

Cambial variants — included concentric phloem or included diffuse phloem or other

8 Mineral inclusions

Prismatic crystals present — in upright and/or square ray cells or in procumbent ray cells or in radial alignment in procumbent ray cells or in chambered upright and/or square ray cells or in non-chambered axial parenchyma cells or in fibres

Druses present in ray parenchyma cells or in axial parenchyma cells or in fibres or in chambered cells Other crystal types — raphides or acicular crystals or styloids and/or elongate crystals or crystals of other shapes or crystal sand

Other diagnostic crystal features — more than one crystal of about the same size per cell or chamber or two distinct sizes of crystal per cell or chamber or crystals in enlarged cells or crystals in tyloses or cystoliths Silica

Chrome Azurol-s test Burning splinter test

In the walnut genus, Juglans, two important microscopic features — crystals in longitudinal parenchyma cells and reticulate thickenings, (called gash-like pits) on some vessel walls — provide a positive separation of North American black walnut (juglans nigra) from European walnut (Juglans regia) (Figure 2.11).

Table 2.2 lists the features used for accurate microscopic identification of hardwoods. These features are extensively described by Brazier and Franklin (1961), Core et al. (1979), Grosser (1977), Hoadley (1990), Panshin and deZeeuw (1980), Phillips (1960), and Schweingruber (1982, 1990). The quarterly bulletin of the International Association of Wood Anatomists (IAWA) provides excellent up-to-date information on microscopic features, reviews of botanical families, book reviews and articles on wood anatomy.

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